This invention relates generally to multi-cavity valve gated injection molding apparatus and more particularly to stack molding apparatus having two elongated valve members reciprocating in alignment to control the melt flow during the injection cycle.
Stack molding apparatus to locate cavities on opposing faces of a movable platen is well known. An example of sprue gated stack molding apparatus is shown in the applicant's U.S. Pat. No. 4,891,001 which issued Jan. 2, 1990, while an example of valve gated stack molding apparatus is shown in the applicant's U.S. Pat. No. 4,212,626 which issued Jul. 15, 1980. While valve gated stack molding has been satisfactory for some applications, it has the disadvantage that some melt is trapped between the faces of the tip ends of the valve members when they close together. This forms a circular piece of plastic film which may stick to one of the valve members or elsewhere in the mold and not drop out when the mold opens for ejection. This piece of plastic film can then prevent proper closure of the mold and/or injection during the next injection cycle.